This invention concerns structures and methods for magnetic image recording. More specifically, this invention concerns magnetic biasing methods and structures for economical image recording in magnetic printing systems.
Machines for producing printed copy form a latent image which is recorded on a magnetic medium are well known to the reproduction arts. Typically, an original image is optically scanned to produce an electrical signal which varies in intensity with the brightness of the original image. The electrical signal, which may for example be stored and regenerated in a computer memory, is applied to magnetic recording heads which produce a sequentially varying magnetic field. The surface of a magnetic recording medium, for example, a drum or an oxide coated tape, moves past the recording heads through the varying magnetic field. A latent magnetic image corresponding to the brightness of the original image is thus recorded on the surface of the magnetic medium. A magnetic ink which may be in the form of particles comprising finely divided ferrogmagnetic powder and a plastic resin, is applied to the surface of the recording medium where it is attracted by the magnetic field variations of the latent image. The ink image is then transferred from the magnetic medium to a final copy material, typically paper, by any of a variety of well-known processes which include electrostatic transfer and pressure transfer. The latent image recorded on the magnetic medium may then be re-inked for printing additional copies or erased to permit the medium to be used for printing a new image.
Typical magnetic printing systems employ a large plurality of magnetic recording heads for separately addressing and recording image points on magnetic recording medium. The recording heads are typically physically small and employ a limited number of conducting turns in an exciting winding. High speed printing operating dictates that the current flow through the exciting windings be in the form of narrow pulses. Large peak head drive currents are typically required to produce a magnetic field capable of saturating the magnetic recording medium with a small number of exciting winding turns.
The cost of circuitry for driving high speed magnetic recording heads generally increases sharply with the peak current required. Large numbers of driver circuits are typically required for magnetic printing applications. It is, therefore, desirable to reduce the peak drive current requirements and therefore the total head current driver cost in magnetic printing applications.
The ferromagnetic ink utilized in magnetic printing applications is attracted by variations in the magnetic field of a latent recorded image. Maximum ink density is, therefore, associated with the image areas incorporating large numbers of magnetic field reversals. Magnetic field reversals may be recorded by applying pulses of opposite polarity to adjacent recording heads in contact with the recording medium. Magnetic field reversals may also be recorded by applying unidirectional pulses to alternate magnetic recording heads to selectively reverse the polarity of a uniformly magnetized medium.
Briefly stated, copending U.S. patent application Ser. No. 571,595 describes an integrated, pin-type recording head array suitable for magnetic printing applications. One pole of the recording head array comprises a sheet of magnetic material while the remaining poles comprise pin cores wrapped with field coils.